Bone Morphogenetic Protein (BMP) receptors are essential for myocyte-dependent functions and signals in cardiac organogenesis. Activin Receptor-Like Kinase-3 (ALK3) is specifically required at mid-gestation for normal development of the trabeculae, compact myocardium, interventricular septum and endocardial cushion. BMPs like BMP2, BMP4 and BMP5, BMP7, BMP10, bind to Serine/threonine kinase receptors, Type-I (ALK3 and ALK6) and Type-II, BMPR2, respectively, and form a heteromeric signaling complex acting in series. In the presence of ligand, the Type-II receptors phosphorylate the Type-I receptors, which activate signaling by intracellular effectors including SMAD transcription factors. TAK1 and cardiac transcription factors such as NKX2.5 and GATA4 play pivotal roles in the cardiogenic BMP signaling pathway. TAK1 is activated by BMPs and also mediates the activity of BMPs. The crosstalk between the SMAD pathway and the TAK1 pathway induces differentiation of cardiac precursor cells. Among the members of SMAD family, SMAD1, SMAD5 and SMAD8 transduce signals from BMPs specifically, while SMAD4 is a general partner of ligand-specific SMADs. Co-overexpression of SMAD1 and SMAD4 induce differentiation of cardiac precursor cells into cardiomyocytes, while overexpression of SMAD6 inhibits differentiation of the cardiac precursor cells into cardiomyocytes. SMAD6 is an inhibitory SMAD that is induced by BMPs and interferes with BMP signaling mediated by SMAD1, SMAD5, and SMAD8.
Initially, BMP2 and/or BMP4 transactivate the expression of two major cardiac-specific transcription factors, namely NKX2.5 and GATA4. This transactivation is mediated by TAK1. Subsequently, NKX2.5 and GATA4 induce differentiation into cardiomyocytes cooperatively with unknown factors induced by Dimethyl sulfoxide (DMSO). The unknown factor(s) induced by DMSO are also required for this step, because neither expression of NKX2.5 and GATA4 nor subsequent terminal differentiation into cardiomyocytes can be induced in the absence of DMSO. Although some cardiac-specific genes, such as MEF2C and MLC2v, are upregulated by NKX2.5 and GATA4 alone, differentiation into beating cardiomyocytes requires the cooperative effects of both NKX2.5 and GATA4. MLC2v and MEF2C are positively regulated by NKX2.5.
ATF2 stimulates the β-MHC promoter activity synergistically with SMAD1, SMAD4 and TAK1 and promotes terminal cardiomyocyte differentiation. ATF2 binds directly to hetero-oligomers of SMADs and is phosphorylated by TGF-β signaling via TAK1 and p38 MAPK. ATF2-induced transactivation of β-MHC gene depends on both the SMAD and the TAK1 pathways. Apart from stimulating β-MHC, ATF2 also plays a pivotal role in transactivation of some cardiac-specific genes such as NPPA and NPPB. NPPB is a major heart secretory product that is considered a clinical marker of the diseased heart and it is also controlled by GATA factors. In humans, the invariable defects in myocardium and AV cushion resulting from conditional deletion of BMP receptors provide strong support for their roles as genes involved in human congenital heart diseases.